add a -Z flag to guarantee that MIR is generated for all functions

[oli-obk]

r? @eddyb

cc @solson

[eddyb]

Could this be -Z instead? To avoid it being insta-stable. LGTM otherwise.

[oli-obk]

done

[eddyb]

@bors r+

[bors]

📌 Commit 87a9ae2 has been approved by eddyb

[eddyb]

@bors r-

[oli-obk]

This now produces a working compiler (stage2 is building right now) without the flag enabled. It also allows me to compile miri and all its dependencies with the flag enabled (not that there's any point, but I didn't want to wait for an entire compiler to build with the flag enabled). So this is basically working now. I know it has no tests yet. I promise I will write them on monday. Would it be possible to r+ this PR without the tests?

[solson]

I successfully built stage2 with the newest code in this PR and the flag hardcoded to true.

[eddyb]

@oli-obk Testing this seems tricky, probably impossible w/o writing a plugin.
@bors r+

[bors]

📌 Commit d74d153 has been approved by eddyb

[bors]

⌛ Testing commit d74d153 with merge d53f39e...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-rustbuild
Approved by: eddyb
Pushing d53f39e to master...